For CHSL application, in the educational qualification section, you should mention your highest completed qualification. Since you passed 12th in 2013, and you are doing B.Tech from 2015 to 2019, if you have not completed B.Tech yet, write “12th Passed” as your qualification. If you completed B.Tech, then mention B.Tech with the passing year. If still studying, you can write “Pursuing B.Tech (2015-2019 batch)” and mention your 12th passing year as well.